Wetting of porous polymeric membranes by liquid absorbents has been considered to be the major problem in membrane gas/liquid contactors (MGLC). In the non-wetted condition, the membrane pores are only filled with the gas. However, partial wetting by the liquid absorbent can result in a significant increase in membrane resistance. 1
The wetting tendency of a membrane by a specific liquid is determined by membrane chemical and morphological properties and is directly proportional to its surface energy and pore size. The reported wettability of some examples of commonly used polymeric membranes with different solutions is summarized in Table 1. It can be seen that due to its lowest surface energy, polytetrafluoroethylene (PTFE) is the most resistant polymer to wetting and therefore it has been extensively used in MGLC systems. However, several serious drawbacks including i) complicated and high cost of fabrication, ii) environmentally malignant method of fabrication (PTFE is typically fabricated via solvent-employed methods), iii) small specific interfacial area leading to a reduction in absorption efficiency, iv) unavailability of small pore sizes, due to the complexity in porosity and pore size control and v) not being recyclable (PTFE is not considered in the category of recyclable polymers), practically limit the large scale utilization of PTFE materials.
TABLE 1MembraneAbsorbentWettabilityReferencePTFEMEA−Nishikawa et al.2Amine solution−Falk Pedersen3(AMP, MEA, MDEA)NaOH solution, MEA−Kim et al.4solutionMEA, AMP−Matsumoto et al.5 -deMontigny et al.6Water, MEA−Khaisri et al.7−PPNaOH solution+Rangwala8Alkanolamine solution+Falk Pedersen 3Amino acid salts+Kumar et al.9(Potassium taurate)NaOH solution, MEA+Kumar et al. 9solutionPropylene carbonate+Matsumoto et al.5Water, MEA+Dindore et al.1MEA, AMP+Khaisri et al.7 -deMontigny et al.6Activated absorbent+Lu et al.10PENaOH solution, MEA+Matsumoto et al.5solutionMEA+Nishikawa et al.2PVDF5% MEA + 5% TEA−Yeon et al.11Water, MEA+Khaisri et al.7
The general characteristics of some kinds of commercial PTFE, polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fiber membranes are presented in Table 2. PP as an inexpensive and readily available alternative has been frequently considered in the literature. However, it has a large wettability and it has been reported to be wetted by alkanolamine solutions in short term operation.1
TABLE 2DescriptionPPPVDFPTFEReferenceMax. available0.250.031.0Lee et al.12pore size (μm)Max. void75.282.270.1Khaisri et al.7fraction (%)82.2—59.2deMontigny et al.6Max. specific285514881340Lee et al.12area (m2/m3)2752—429deMontigny et al.6Cost (US $/m)0.01—23deMontigny et al.60.010.3611.5Khaisri et al.7
Polyethylene (PE), having the same wetting tendency as PP, can be another alternative which has been of little consideration in the literature. However, PE can be an interesting option due to its several advantages including recyclability, flexibility of operation, simplicity of transformation to flat and hollow fiber membranes through different methods, frequent availability and inexpensive price, chemical stability higher than that of PP (due to less side groups on its molecular chains)13 and the possibility of different hydrophobic modifications to diminish surface wettability. On the other hand, methods used for the hydrophobic treatment of polyolefins such as laser etching14, plasma treatment15 and solution casting16 are mostly uneconomic and unsafe, since they use specific installations, toxic solvents and complicated procedures. In addition, most of these methods cannot be applied on non-flat surfaces and thus are of no use for hollow fiber membranes.
It has been shown that effective roughening of a specific surface can significantly increase its hydrophobicity16,17,18,19. For example, many natural plant leaves have super hydrophobic properties due to their rough surface structure20. There have also been several attempts reported in the literature to produce artificial super hydrophobic surfaces via roughening low surface energy materials18,21. However, despite its important role in the reduction of membrane wettability, increasing the hydrophobicity of porous polymeric membranes via creating rough structures remains untouched.
Currently, porous polymeric membranes are frequently fabricated via a conventional wet-phase inversion method through thermally induced phase separation (TIPS)22 or non-solvent induced phase separation (NIPS)23 processes. These processes mainly involve the dissolution of polymer in toxic organic solvents. The porous structure forms during phase separation that is induced either by thermally cooling the polymer solution or by adding a non-solvent. A serious drawback of these processes is the use of a large amount of expensive, harmful and partly flammable solvents which have to be removed after membrane formation via several intense washing procedures, making the process noneconomic and environmentally harmful. Besides the above mentioned disadvantages, these processes suffer from low production rates, due to the slow liquid-liquid phase separation phenomena24.
Only a few solvent-free methods for fabrication of porous polymeric membranes have been reported in the literature.25,26,27 The most important solvent-free membrane manufacturing technique is the melt-spinning and stretching method. This technique is based on the melt extrusion of pure semi-crystalline polymers to form flat or hollow fiber precursors, followed by axial stretching of such precursors to form a porous structure. However, this technique is only applicable to semi-crystalline polymers and in addition to the mechanical stretching process, it requires several thermal post-treatments to promote the crystallinity and to avoid the membrane shrinkage28.
There are few studies in the literature concerning the fabrication of open-cell foams containing polymer/filler composites. R. K. M. Chu et al.29 fabricated open-cell PP foams for sound absorption applications, introducing NaCl of 106-850 μm. The foam products were then leached in water for at least 96 h to dissolve away the salts. Narkis and Joseph30 studied the tensile properties of poly(methyl methacrylate) (PMMA), polysulfone (PSF) and polystyrene (PS) foams produced by salt extraction. Recently, Dangtungee and Supaphol31 investigated the rheological properties of low-density polyethylene (LDPE) and sodium chloride (NaCl) mixtures of varying particle size (i.e., 45, 75, and 125 μm) in the range of 5-25 wt. %. Verdolotti et al.32 studied the effect of the incorporation of several lithium salts (LiCl, LiClO4 and LiCF3SO3) on the electrical and mechanical properties of polyurethane rigid (PUR) foams. However, despite the importance of porous polymeric products, there has been little concentration on the employment of open-cellular foams, prepared via leaching the salts from a polymer/salt composite matrix, for membrane applications. Moreover, the leaching process, the most important part of this technique, still remains noticeably untouched. Long leaching durations (around 90 h) and a batch wise leaching process as a post treatment make the reported processes industrially inapplicable. Further, none of the abovementioned studies concentrated on the control of salt particle size, salt content and the final product shape (only film products were prepared). In addition, inhomogeneity and the large particle size of the salts used (45-850 μm) makes the reported foams inappropriate for membrane applications.